Virulence loss and amastigote transformation failure determine host cell responses to Leishmania mexicana.

The effect of alterations in virulence and transformation by long-term in vitro culture of Leishmania mexicana promastigotes on infectivity and immune responses was investigated. Fresh parasite cultures harvested from Balb/c mice were passaged 20 times in vitro. Infectivity was decreased and was completely avirulent after 20 passages.